Dawn
The mission that confirmed the howardites, eucrites and diogenites come from the giant asteroid Vesta.
Key facts
- Agency
- NASA (United States)
- Type of mission
- Orbiter of two worlds
- Target
- Asteroid 4 Vesta and dwarf planet Ceres
- Launched
- 27 September 2007
- Status
- Ended 2018
Dawn was the first spacecraft to orbit two different worlds beyond the Earth and Moon. It used ion engines, which push gently but for years on end, to spiral into orbit around Vesta and then travel on to Ceres, the two largest bodies in the asteroid belt.
Vesta
Vesta is about 525 kilometres across. In the 1970s telescopes had shown that its reflected light matches a family of meteorites: the eucrites, which are basalts; the diogenites, made largely of the pyroxene orthopyroxene; and the howardites, which are breccias mixing the two. Together they are called the HED meteorites.
Dawn orbited Vesta from July 2011 to September 2012. It mapped a huge impact basin near the south pole, Rheasilvia, about 500 kilometres wide, with a central peak more than twice as high as Mount Everest. The impact that dug it, about a billion years ago, threw out enough rock to explain the HED meteorites and the small Vesta-like asteroids, the vestoids, that carry such rock towards Earth.
Dawn's gamma-ray and neutron detector measured the surface elements and found them to match the HED meteorites, especially the howardites, which are mixtures of surface rock, just as expected. Its gravity measurements showed Vesta has an iron core about 220 kilometres across, as the meteorites predicted.
Ceres
At Ceres, about 940 kilometres across, Dawn found bright spots of sodium carbonate salts in Occator crater, clays containing ammonia and a lonely ice volcano, Ahuna Mons. Ceres is a wet, salty world, quite unlike Vesta.
Key dates
| Date | What happened |
|---|---|
| 27 September 2007 | Dawn launched on a Delta II rocket. |
| 17 February 2009 | Flew past Mars for a gravity assist. |
| 16 July 2011 | Entered orbit around Vesta. |
| 5 September 2012 | Left Vesta for Ceres. |
| 6 March 2015 | Entered orbit around Ceres. |
| 1 November 2018 | The mission ended when Dawn ran out of fuel for pointing; it remains in orbit around Ceres. |
See where this mission fits among all the others in the mission timeline.
Did you know?
About one in every 16 meteorites seen to fall is an HED meteorite from Vesta.
Dawn's ion engines changed its speed by about 11 kilometres per second over the whole mission, a record for a spacecraft's own engines at the time.